Patent GB-1526834-A (GB)
Title: Exposure control device for a camera
IPC: G03B7/093
Filed: 1975-11-24
Abstract: 1526834 Camera shutter control ASAHI KOGAKU KOGYO KK 24 Nov 1975 [26 Nov 1974 15 Aug 1975] 48234/75 Heading G2A A shutter control arrangement generates and counts prior to shutter opening a number of pulses proportional to light intensity, generates and counts upon shutter opening a train of timing pulses, compares the two counts, and closes the shutter when they are equal; the light dependent pulses (and preferably the timing pulses as well) being counted in a digital memory in a decimal manner. Preferably if the count is very low or very high (indicating over or under exposure) the diaphragm aperture is automatically altered by a predetermined amount, the system reset and the exposure determining process repeated. As shown, upon camera release an oscillator 57 produces a pulse which steps on a ring counter 60 to open an AND gate 32 until the next pulse. While AND gate 32 is open pulses pass through it from a photoresponsive pulse generator 31 to a decimal counter 2. Counter 2 comprises four ring counters 2a, b, c, d which respectively record units, 10's, 100's and 1,000's. If the count in counter 2 is less than 10, a discriminator 34 energizes a diaphragm closing circuit 46 and resets the system for a repeat operation. If the count exceeds 299, a discriminator 43 energizes a diaphragm opening circuit 51 and resets the system. When the count is from 10 to 299, a discriminator 39, via an AND gate 48, turns on a thyristor 52 to energize a shutter opening electromagnet 53. Simultaneously a switch 4 is closed so that timing pulses from a generator 3 pass to a decimal counter 5. When the counts in ring counters Sa to d coincide with those in ring counters 2a to d, coincidence detectors 6a to d reset ring counters 2a to d. Thus OR gates 33 a to d are all at "O" and a NOR gate 54 switches to "I" to turn on a thyristor 8 which energizes a shutter closing electromagnet 9. As an alternative, binary-coded decimal counters may be used. If an exposure time exceeding 30ms is required, a switch 49 is changed over to disable discriminator 43. Fig. 1 (not shown) shows a simpler version of Fig. 6 and includes LED's (La, Lb, Lc) for indicating shutter time. In Figs. 2 and 7 (not shown) a single reversible counter replaces the two counters 2 and 6 of Fig. 6. In Figs. 3 and 8 (not shown) the count in counter 2 is transmitted to a subtraction counter (14) to which the timing pulses are fed, the shutter being closed when the count in the subtraction counter is zero. Figs. 4 and 9 (not shown) show details of a generator which includes a CdS cell or a phototransistor to produce pulses whose frequency depends on object brightness, adjustment of resistance or frequency division being used to account for film speed and diaphragm aperture. Fig. 10 (not shown) shows a photoresponsive multi-vibrator which generates a photodependent gate pulse during which fixed frequency timing pulses are counted, adjustment of resistance or capacitance being used to take account of film speed and diaphragm aperture. Fig. 11 (not shown) shows a diaphragm setting ring (86) operated by a pulsed stepper motor (83), and a lamp (90) which lights up during adjustment of ring (86). Fig. 12 (not shown) shows a diaphragm setting ring (86) and an additional ring (93) for pre-setting a limit on diaphragm opening to limit the reduction in depth of field. When the limit is reached, a lamp (96) goes on to indicate the danger of camera shake.